Abstract
5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), 8-OH-DPAT and RU 24969 relax the guinea-pig ileum precontracted with histamine. This relaxation was characterized using 5-CT as agonist with a series of 8 competitive antagonists. [ 125I]LSD binding was measured in a membrane preparation of the longitudinal muscle of the guinea-pig ileum in the presence of 3 × 10 −7 M cinanserin in order to suppress binding to 5-HT 2 receptors. There was a significant correlation (P < 0.01) between the antagonism of 5-CT-induced relaxation (pA 2 values) and the affinity values of the antagonists for [ 125I]LSD binding (pK D values). It was also shown that 5-CT relaxed histamine-precontracted longitudinal muscle strips of the guinea-pig ileum. The results suggest that the 5-HT receptor-mediating relaxation in the guinea-pig ileum can be labelled with [ 125I]LSD and that this receptor does not belong to the 5-HT 2, 5-HT 1A, 5-HT 1B or 5-HT 1C receptor subtypes.
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